Pilot signal transmission method and mobile communication system

ABSTRACT

A method of transmitting pilot signals for channel compensation in a mobile communication system based on DFT-spread-OFDM is disclosed. In the disclosed method, a first pilot signal and a second pilot signal are time-division multiplexed together with a data signal of a user into a time-division multiplexed signal which data signal is assigned a certain bandwidth; the time-division multiplexed signal is frequency-division multiplexed together with time-division multiplexed signals of other users when wirelessly transmitted; and the first pilot signal is assigned a bandwidth larger than the bandwidth of the data signal and the second pilot signal is assigned a bandwidth smaller than the bandwidth of the data signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a pilot signal transmissionmethod and a mobile communication system, and more particularly relatesto a pilot signal transmission method and a mobile communication systemwhere a pilot signal for channel compensation is time-divisionmultiplexed together with a data signal of a user, which data signal isassigned a certain bandwidth and to be wirelessly transmitted based onDFT-spread-OFDM, into a time-division multiplexed signal, thetime-division multiplexed signal is frequency-division multiplexedtogether with time-division multiplexed signals of other users into afrequency-division multiplexed signal, and the frequency-divisionmultiplexed signal is transmitted.

2. Description of the Related Art

In the uplink of the Evolved UTRA (enhanced system of W-CDMA) standardregarding the enhancement of the next generation mobile communicationsystem, the use of DFT-spread-OFDM (orthogonal frequency divisionmultiplexing) is being discussed as a wireless access technology. InDFT-spread-OFDM, a data signal of a user is assigned a certain frequencybandwidth (resource unit) and frequency-division multiplexed togetherwith data signals of other users. DFT-spread-OFDM employs asingle-carrier transmission method instead of a multi-carriertransmission method employed in, for example, OFDM and thereforeprovides a lower peak-to-average power ratio (PAPR). Also,DFT-spread-OFDM employs frequency-domain signal processing and thereforemakes it possible to flexibly arrange components of a single-carriersignal in the frequency domain.

FIGS. 11A and 11B are drawings illustrating exemplary signal mappings inthe frequency domain. In FIGS. 11A and 11B, a resource block (RB)indicates a minimum unit of frequency bandwidth assignable to a mobilestation within the entire frequency bandwidth of a system. In alocalized mapping shown in FIG. 11A, adjacent subcarriers (SC) arecombined as an RB. In a distributed mapping shown in FIG. 11B, SCs apartfrom each other are combined as an RB. In either case, mobile stationsuse different RBs and therefore multiuser interference within a cell canbe effectively avoided; in other words, frequency use efficiency ishigh.

In the Evolved UTRA standard, the use of the Zadoff-Chu sequence(hereafter called ZC sequence), a variation of the constant amplitudeand zero auto correlation (CAZAC) sequence, for the pilot signal isbeing considered. When the sequence length N is an odd number, a ZCsequence is expressed by the following formula (1):

$\begin{matrix}{{c_{k}(n)} = {{\exp \left\lbrack {{- \frac{{j2}\; \pi \; k}{N}}\left( {{qn} + {n\frac{n + 1}{2}}} \right)} \right\rbrack}\mspace{45mu} \left( {{n = 0},\ldots \mspace{11mu},{N - 1}} \right)}} & (1)\end{matrix}$

When the sequence length N is an even number, a ZC sequence is expressedby the following formula (2):

$\begin{matrix}{{c_{k}(n)} = {{\exp \left\lbrack {{- \frac{{j2}\; \pi \; k}{N}}\left( {{qn} + \frac{n^{2}}{2}} \right)} \right\rbrack}\mspace{45mu} \left( {{n = 0},\ldots \mspace{11mu},{N - 1}} \right)}} & (2)\end{matrix}$

In the above formulas, q indicates an integer and k indicates a sequencenumber.

When designing a pilot signal, multicell interference must first betaken into account. In a multicell environment, instead of multiplyingZC sequences with a scramble code unique to each cell, a set of ZCsequences with low cross-correlation are generated and assigned to thepilot signals of each cell. In this case, to maximize the number ofsequences k in the set of ZC sequences and thereby to maximize theflexibility in assigning ZC sequences to each cell, the sequence lengthN must be a prime number. Secondly, to obtain channel estimates for thefrequency band assigned to a data signal, a pilot signal is preferablyassigned the same bandwidth as that of the data signal.

The ZC sequence is a variation of the CAZAC sequence and therefore hasconstant amplitude in the time domain and the frequency domain and zeroautocorrelation except when its phase difference is 0. Therefore, usingZC sequences as pilot signals makes it possible to keep the PAPR of atransmitted signal substantially low. Also, using ZC sequences as pilotsignals makes it possible for a receiving end to substantially reducethe fluctuation in the SNR of channel estimates in the frequency domainbetween subcarriers.

When using ZC sequences as pilot signals, if the pilot signals aremultiplied with a scramble code unique to each cell as in the case ofdata signals, the properties of the ZC sequences that are peculiar to aCAZAC sequence variation are lost. Therefore, when using ZC sequences aspilot signals for uplink transmission in a cellular system, rather thanmultiplying ZC sequences with a scramble code unique to each cell, it ispreferable to generate multiple ZC sequences with low cross-correlationby changing the sequence number k and to assign the generated ZCsequences to each cell.

To improve the flexibility (number of replications) in assigning ZCsequences to each cell, it is preferable to be able to efficientlygenerate ZC sequences with low cross-correlation. It is known that whenthe sequence length N is a prime number, N−1 number of ZC sequences withlow cross-correlation can be generated. For this reason, it is proposedto use ZC sequences having prime sequence lengths N as pilot signals foruplink transmission in a cellular system.

FIG. 12 is a drawing illustrating an exemplary mapping of pilot and datasignals in the time and frequency domains and used to describe problemsin a conventional technology. In FIG. 12, for brevity, it is assumedthat the intervals of subcarriers for pilot signals and data signals arethe same. Data signal and pilot signal regions are time-divisionmultiplexed and one pilot signal region is provided before and after onedata signal region. According to RB assignment information, RB1 and RB2are assigned to the data signal region of mobile station A and the samefrequency band as that of the data signal region is assigned to thepilot signal regions.

[Non-patent document 1] “Multiplexing Method for Orthogonal ReferenceSignals for E-UTRA Uplink” Agenda Item: 11.2.1, R1-061193, 3GPP TSG-RANWG1 Meeting No. 45, Shanghai, China, 8-12 May, 2006

However, while the number of subcarriers for a data signal is anintegral multiple of the number of subcarriers in an RB, the number ofsubcarriers (sequence length N of a ZC sequence) for a pilot signal mustbe a prime number as described above. Therefore, the frequencybandwidths used by a data signal and a pilot signal are basicallydifferent. Also, it is necessary to prevent interference between pilotsignals of mobile stations that use adjacent RBs.

One way to solve the above problems is to set the number of subcarriers(sequence length of a ZC sequence) for a pilot signal to the largestprime number within the bandwidth of a data signal. In FIG. 12, 13subcarriers, where 13 is the largest prime number within the frequencybandwidth 16 (unit of bandwidth is omitted here) of the data signal, areassigned to each of the pilot signals. In this case, however, the pilotsignals do not cover the frequency bands corresponding to both ends ofthe data signal. Therefore, the channel estimates for the one rightmostsubcarrier and the two leftmost subcarriers of the data signal must beextrapolated.

There are two major factors that affect the accuracy of channelestimation at a receiving unit of a base station. The first factor isthermal noise components and interference signal components contained ina received signal. The second factor is the accuracy ofinterpolation/extrapolation by a time and frequencyinterpolation/extrapolation unit. Channel distortion becomes greater inthe time direction in proportion to the traveling speed of a mobilestation and in the frequency direction in proportion to the delayspread. When channel distortion is low, interpolation/extrapolation canbe performed accurately to a certain extent. However, since channeldistortion occurs in a very complicated manner, it is difficult toaccurately perform interpolation/extrapolation when channel distortionis high. Also, in urban areas, normally, channel distortion is greaterand more complicated in the frequency direction than in the timedirection, and an extrapolation method provides less accurate resultsthan an interpolation method.

As described above, when a ZC sequence with a prime number sequencelength is used as a pilot signal for uplink transmission, the bandwidthof the pilot signal may become different from that of a data signal,making it necessary to extrapolate channel estimates in the frequencydirection. This, in turn, greatly reduces the accuracy of channelestimation and thereby degrades the reception characteristics of a datasignal. On the other hand, making the bandwidth of a pilot signal largerthan that of a data signal may cause interference between pilot signalsof different users.

SUMMARY OF THE INVENTION

The present invention provides a pilot signal transmission method and amobile communication system that substantially obviate one or moreproblems caused by the limitations and disadvantages of the related art.

Embodiments of the present invention provide a pilot signal transmissionmethod and a mobile communication system that enable accurate channelestimation.

An embodiment of the present invention provides a method of transmittingpilot signals in a mobile communication system, which method includesthe steps of time-division multiplexing a first pilot signal and asecond pilot signal for channel compensation together with a data signalof a user, which data signal is assigned a certain bandwidth and to bewirelessly transmitted based on DFT-spread-OFDM, into a time-divisionmultiplexed signal; frequency-division multiplexing the time-divisionmultiplexed signal together with time-division multiplexed signals ofother users into a frequency-division multiplexed signal; andtransmitting the frequency-division multiplexed signal; wherein thefirst pilot signal is assigned a bandwidth larger than the bandwidthassigned to the data signal and the second pilot signal is assigned abandwidth smaller than the bandwidth assigned to the data signal; andthe first pilot signal and the second pilot signal are multiplexed alonga time axis.

Another embodiment of the present invention provides a method oftransmitting pilot signals in a mobile communication system, whichmethod includes the steps of time-division multiplexing a first pilotsignal and a second pilot signal for channel compensation together witha data signal of a user, which data signal is assigned a certainbandwidth and to be wirelessly transmitted based on DFT-spread-OFDM,into a time-division multiplexed signal; frequency-division multiplexingthe time-division multiplexed signal together with time-divisionmultiplexed signals of other users into a frequency-division multiplexedsignal; and transmitting the frequency-division multiplexed signal;wherein each of the first pilot signal and the second pilot signal iscomposed of a sequence having a sequence length that is the largestprime number within the bandwidth assigned to the data signal; the firstpilot signal is assigned a frequency band starting from one end of thebandwidth assigned to the data signal; the second pilot signal isassigned a frequency band starting from the other end of the bandwidthassigned to the data signal; and the first pilot signal and the secondpilot signal are multiplexed alternately along a time axis.

Another embodiment of the present invention provides a method oftransmitting a pilot signal in a mobile communication system, whichmethod includes the steps of time-division multiplexing a pilot signalfor channel compensation together with a data signal of a user, whichdata signal is assigned a certain bandwidth and to be wirelesslytransmitted based on DFT-spread-OFDM, into a time-division multiplexedsignal; frequency-division multiplexing the time-division multiplexedsignal together with time-division multiplexed signals of other usersinto a frequency-division multiplexed signal; and transmitting thefrequency-division multiplexed signal; wherein the pilot signal iscomposed of a sequence having a sequence length that is a prime number;and the bandwidth assigned to the data signal is adjusted to match abandwidth corresponding to the sequence length.

Another embodiment of the present invention provides a mobilecommunication system including a base station and at least one mobilestation where a pilot signal for channel compensation is time-divisionmultiplexed together with a data signal of the mobile station, whichdata signal is assigned a certain bandwidth and to be wirelesslytransmitted between the base station and the mobile station based onDFT-spread-OFDM, into a time-division multiplexed signal, thetime-division multiplexed signal is frequency-division multiplexedtogether with time-division multiplexed signals of other mobile stationsin the mobile communication system into a frequency-division multiplexedsignal, and the frequency-division multiplexed signal is transmitted,wherein the base station is configured to transmit information over adownlink control channel to the mobile station which informationincludes a bandwidth to be assigned to the data signal of the mobilestation; the mobile station is configured to multiplex a first pilotsignal composed of a sequence having a sequence length that is thesmallest prime number exceeding the bandwidth assigned to the datasignal of the mobile station and a second pilot signal composed of asequence having a sequence length that is the largest prime numberwithin the bandwidth assigned to the data signal of the mobile stationalternately along a time axis in such a manner that either of the firstpilot signal and the second pilot signal does not overlap a pilot signaltransmitted at substantially the same timing from an adjacent one of theother mobile stations a data signal of which adjacent one of the othermobile stations is assigned a frequency band adjacent to that assignedto the data signal of the mobile station.

Still another embodiment of the present invention provides a mobilecommunication system including a base station and at least one mobilestation where a pilot signal for channel compensation is time-divisionmultiplexed together with a data signal of the mobile station, whichdata signal is assigned a certain bandwidth and to be wirelesslytransmitted between the base station and the mobile station based onDFT-spread-OFDM, into a time-division multiplexed signal, thetime-division multiplexed signal is frequency-division multiplexedtogether with time-division multiplexed signals of other mobile stationsin the mobile communication system into a frequency-division multiplexedsignal, and the frequency-division multiplexed signal is transmitted,wherein the base station is configured to transmit information over adownlink control channel to the mobile station which informationincludes a bandwidth to be assigned to the data signal of the mobilestation and a priority to determine a sequence length of the pilotsignal; and the mobile station is configured to, when the priority ishigh, multiplex a first pilot signal composed of a sequence having asequence length that is the smallest prime number exceeding thebandwidth assigned to the data signal of the mobile station.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an exemplary configuration of anexemplary receiving unit of a base station according to a firstembodiment of the present invention;

FIG. 2 is a block diagram illustrating an exemplary configuration of anexemplary transmitting unit of a mobile station according to the firstembodiment of the present invention;

FIG. 3 is a drawing illustrating an exemplary mapping of pilot signalsaccording to an embodiment of the present invention;

FIG. 4 is a flowchart showing an exemplary process of determining thesequence length of a pilot signal according to an embodiment of thepresent invention;

FIGS. 5A and 5B are drawings illustrating other exemplary mappings ofpilot signals according to embodiments of the present invention;

FIG. 6 is a drawing illustrating another exemplary mapping of pilotsignals according to an embodiment of the present invention;

FIGS. 7A and 7B are drawings illustrating other exemplary mappings ofpilot signals according to embodiments of the present invention;

FIG. 8 is a block diagram illustrating an exemplary configuration of anexemplary receiving unit of a base station according to a secondembodiment of the present invention;

FIG. 9 is a block diagram illustrating an exemplary configuration of anexemplary transmitting unit of a mobile station according to the secondembodiment of the present invention;

FIGS. 10A and 10B are drawings used to describe the deficiency rate of abandwidth usable by a pilot signal;

FIGS. 11A and 11B are drawings illustrating exemplary signal mappings inthe frequency domain; and

FIG. 12 is a drawing used to describe problems in a conventionaltechnology.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 1. First Embodiment

Preferred embodiments of the present invention are described below withreference to the accompanying drawings. In the drawings, the samereference numbers are used for the same or corresponding parts. FIG. 1and FIG. 2 are drawings illustrating an exemplary mobile communicationsystem based on DFT-spread-OFDM according to a first embodiment of thepresent invention.

FIG. 1 is a block diagram illustrating an exemplary configuration of anexemplary receiving unit of a base station 10. An uplink radio signalfrom a mobile station 50 is quadrature-demodulated by a reception RFunit 11 and thereby converted into a baseband signal, and the basebandsignal is converted from analog to digital by an A/D converting unit 12.A path search unit 14 performs a correlation calculation in the timedomain between the A/D converted signal (received signal) and a replicaof a transmitted pilot signal and thereby detects a reception timing(starting point of an effective signal component) of each path. A CPremoving unit 13 removes cyclic prefixes (CP) from the received signalbased on the reception timing of path 1 and thereby extracts theeffective signal component. A data-pilot signal separating unit 15separates pilot signals and a data signal that are time-divisionmultiplexed in the received signal.

The pilot signals are converted from the time domain into the frequencydomain by an FFT unit 16 b and sent to one of demodulating circuits 40A,40B, . . . each of which handles one mobile station at a time. In thedemodulating circuit 40A, for example, a sequence length determiningunit 41 determines the sequence length of each of the pilot signals andits mapping to subcarriers based on the current RB assignmentinformation of the mobile station 50 and a pilot signal frequency bandassignment rule described later. Based on the determined information, apilot signal generating unit 25 generates a replica of the transmittedpilot signal in the time domain and a DFT calculation unit 26 convertsthe time domain pilot signal into a frequency domain pilot signal. Then,a subcarrier mapping unit 27 maps the frequency domain pilot signal ontosubcarriers assigned to the mobile station 50.

A channel estimation unit 17 performs a correlation calculation in thefrequency domain between the received pilot signal and the replica ofthe transmitted pilot signal for each of the subcarriers where thefrequency domain pilot signal is mapped and thereby estimates thechannel distortion in the frequency domain for each of the subcarriers.

An SIR estimation unit 28 estimates reception SIRs for the RBs assignedto the data signal based on the channel estimates obtained by thechannel estimation unit 17. In estimating the reception SIR for an RB,the channel estimate of subcarriers corresponding to the RB is used. Achannel estimate is expressed by a complex number. “S” in SIR indicatesa desired signal power and is obtained by adding the square of the realpart of the channel estimate and the square of the imaginary part of thechannel estimate. “I” in SIR indicates an interference signal power andis the variance of multiple symbols. A reception SIR estimate is theratio of the desired signal power S to the interference signal power I.The SIR estimation unit 28 also calculates a noise power estimate usedby a weighting factor generating unit 20 described later. Morespecifically, the SIR estimation unit 28 calculates a noise powerestimate by averaging the obtained interference signal powers I of theRBs assigned to the data signal.

An RB assignment unit 31 assigns RBs (generates the next RB assignmentinformation) used for the next transmission of a data signal from themobile station 50 based on the obtained reception SIR estimates. Forexample, the RB assignment unit 31 assigns RBs having reception SIRestimates greater than a certain threshold. Assigning RBs with highreception quality results in improving the throughput of an entire cell.A control signal modulating unit 32 maps the next RB assignmentinformation obtained by the RB assignment unit 31 onto a control signaland feeds the control signal back to the mobile station 50.

The next RB assignment information is used as the current RB assignmentinformation by the mobile station 50 when sending a next data signal.Also, the next RB assignment information is used as the current RBassignment information by the base station 10 when receiving the nextdata signal. The next RB assignment information generated by the RBassignment unit 31 is delayed by a buffer 30 and thereby is used as thecurrent RB assignment information by the base station 10.

When interpolation/extrapolation of the channel estimates is necessary,a time and frequency interpolation/extrapolation unit 18interpolates/extrapolates in the time and frequency directions (forexample, by linear interpolation) the channel estimates of some of thesubcarriers (FFT blocks) in a subframe, which channel estimates areobtained by the channel estimation unit 17, and thereby obtains thechannel estimates of all of the subcarriers (FFT blocks) in thesubframe.

The weighting factor generating unit 20 calculates an MMSE weight usedby a frequency equalization unit 19. For example, when the channelestimate is H and the noise power estimate is N, the MMSE weight W for asubcarrier or an FFT block is obtained by the following formula (3):

$\begin{matrix}{W = \frac{H^{*}}{{H}^{2} + N}} & (3)\end{matrix}$

In formula (3), the symbol * indicates a complex conjugate.

The received data signal is converted from the time domain into thefrequency domain by an FFT unit 16 a and sent to one of demodulatingcircuits 40A, 40B, . . . each of which handles one mobile station at atime. In the demodulating circuit 40A, for example, the output from theFFT unit 16 a is frequency-equalized by the frequency equalization unit19. More specifically, the FFT unit 16 a multiplies the received datasignal with the corresponding MMSE weight for each subcarrier or FFTblock.

An effective subcarrier determining unit 29 determines the positions ofeffective subcarriers used by the data signal based on the current RBassignment information. The RB assignment information is obtained fromthe RB assignment unit 31 via the buffer 30. A subcarrier demapping unit21 extracts the data signal in the RBs assigned to the mobile station 50from the received signal in the FFT blocks. An IDFT calculation unit 22converts the data signal from the frequency domain to the time domainand a data demodulating unit 23 demodulates the converted data signal.Then, a turbo decoder 24 performs error-correction decoding on thedemodulated data signal and thereby obtains a decoded data signal. Thedemodulating circuit 40B also functions in the same manner as describedabove.

FIG. 2 is a block diagram illustrating an exemplary configuration of anexemplary transmitting unit of a mobile station. A downlink radio signalfrom the base station 10 is received and converted into a basebandsignal by a reception RF unit 63. A control signal demodulating unit 64demodulates the control signal fed back from the base station 10 andextracts the next RB assignment information including the number ofassigned RBs and RB numbers. The next RB assignment information fed backfrom the base station 10 is used as the current RB assignmentinformation by the mobile station 50 when sending the next data signal.

A turbo encoder 51 performs error-correction encoding on a transmissiondata signal of the mobile station 50 and a data modulating unit 52modulates the encoded data signal. A scramble code multiplying unit 53multiplies the modulated data signal with a scramble code that is uniqueto each cell to reduce inter-cell interference. A DFT calculation unit54 a performs discrete Fourier transform (DFT) on the multiplied datasignal symbol by symbol according to the number of assigned RBs andthereby converts the multiplied data signal from the time domain to thefrequency domain. For example, when the number of subcarriers in an RBis Nc and the number of assigned RBs is NRB, a symbol is expressed byNc×NRB. A subcarrier mapping unit 55 a maps the data signal output fromthe DFT calculation unit 54 a onto subcarriers in a localized mappingmanner based on the RB assignment information in the frequency domain.An IFFT calculation unit 56 a converts the mapped data signal from thefrequency domain back again to the time domain. A CP inserting unit 57 ainserts a cyclic prefix (CP) between samples (IFFT blocks) output fromthe IFFT calculation unit 56 a.

A sequence length determining unit 71 determines the sequence length ofa pilot signal and the mapping of the pilot signal to subcarriers basedon the RB assignment information from the base station 10 and the pilotsignal frequency band assignment rule that is the same as that of thebase station 10. Based on the determined information, a pilot signalgenerating unit 61 generates a pilot signal in the time domain and a DFTcalculation unit 54 b converts the time domain pilot signal into afrequency domain pilot signal. Then, a subcarrier mapping unit 55 b mapsthe frequency domain pilot signal onto the subcarriers assigned to themobile station 50. The exemplary transmitting unit of the mobile station50 may be configured to periodically transmit a pilot signal for use bythe base station 10 to measure a channel quality indicator (CQI) of eachRB. In this case, the subcarrier mapping unit 55 b maps the frequencydomain pilot signal output from the DFT calculation unit 54 b over theentire frequency band in a distributed mapping manner. An IFFTcalculation unit 56 b converts the mapped pilot signal from thefrequency domain back again to the time domain. A CP inserting unit 57 binserts a CP between samples output from the IFFT calculation unit 56 b.

A data-pilot signal multiplexing unit 58 time-division multiplexes theobtained data and pilot signals into a time-division multiplexed signaland a D/A conversion unit 59 converts the time-division multiplexedsignal from digital to analog. Then, a transmission RF unit 60 performsquadrature modulation on the converted signal, in other words, convertsthe baseband signal into a radio frequency signal, and transmits theradio frequency signal from a transmitting antenna.

As described above, in the exemplary mobile communication system, a datasignal and pilot signals for channel compensation are time-divisionmultiplexed into a time-division multiplexed signal, and thetime-division multiplexed signal is frequency-division multiplexed withtime-division multiplexed signals from other users when wirelesslytransmitted. In this case, downlink signals from the base station 10 tothe mobile stations 50 are frequency-division multiplexed in the basestation 10 and uplink signals from the mobile stations 50 to the basestation 10 are frequency-division multiplexed in the air.

FIG. 3 is a drawing illustrating an exemplary mapping of pilot signalsaccording to an embodiment of the present invention. In FIG. 3, thefrequency bandwidth (32) of a system (system bandwidth) is divided intothree frequency bands used by three mobile stations A through C. As ageneral rule, the sequence length of a pilot signal is preferablydetermined so that the pilot signal uses the bandwidth corresponding tothat assigned to a data signal of the user as much as possible and thepilot signal does not overlap (interfere with) another pilot signal ofan adjacent data signal (user). Also, the sequence length of a pilotsignal is preferably determined so as not to exceed the systembandwidth. In this embodiment, a pilot signal has constant signalamplitude in the time and frequency domains and zero autocorrelationexcept when its phase difference is 0. For example, a pilot signal maybe composed of a sequence of frequency components obtained by discreteFourier transforming a Zadoff-Chu sequence having a prime sequencelength. The Zadoff-Chu sequence is a variation of the constant amplitudeand zero auto correlation (CAZAC) sequence. An exemplary mapping ofpilot signals is described below.

In FIG. 3, RB1 (subcarriers (SCs) f1 through f8) is assigned to user A,RB2 and RB3 (SCs f9 through f24) are assigned to user B, and RB4 (SCsf25 through f32) is assigned to user C. A sequence length of 7 that isthe largest prime number within the bandwidth (8) of the data signal ofuser A is assigned to the leading pilot signal 1 of user A, and asequence length of 11 that is the smallest prime number exceeding thebandwidth (8) of the data signal is assigned to the succeeding pilotsignal 2. Symbol N (narrow) indicates the narrower bandwidth assigned toone of the pilot signals of a user and symbol W (wide) indicates thewider bandwidth assigned to the other one of the pilot signals.Exemplary prime numbers are shown in FIG. 10B.

Sequence lengths assigned to the pilot signals of adjacent user B arecomplementary to those assigned to the pilot signals of user A. Asequence length of 17 (W17) that is the smallest prime number exceedingthe bandwidth (16) of the data signal of user B is assigned to theleading pilot signal 1 of user B, and a sequence length of 13 (N13) thatis the largest prime number within the bandwidth (16) of the data signalis assigned to the succeeding pilot signal 2.

The sum of the sequence lengths (N7 and W17) assigned to the leadingpilot signals 1 of users A and B is 24 and equals the sum of the datasignal wavelengths (8 and 16) of users A and B. Also, the sum of thesequence lengths (W11 and N13) assigned to the succeeding pilot signals2 of users A and B is 24 and equals the sum of the data signalwavelengths (8 and 16) of users A and B. Accordingly, the pilot signalsof users A and B mapped as described above do not interfere with eachother and do not even interfere with pilot signals of other users.

Also, with the exemplary mapping shown in FIG. 3, the channel estimationfor SC f8 of the data signal of user A can be accurately performed byusing the succeeding pilot signal 2 (W11) of user A, and the channelestimation for SCs f9-f11 of the data signal of user B can be accuratelyperformed by using the leading pilot signal 1 (W17) of user B.

The mapping of pilot signals of user C can be started from scratch sincethe bandwidth (24) corresponding to that of the data signals of users Aand B is filled with their pilot signals. However, since the frequencyband of the data signal of user C is positioned at the rightmost part ofthe system bandwidth, the sequence lengths of pilot signals of user Ccannot exceed the data signal bandwidth of 8. For this reason, asequence length of 7 (N7) that is the largest prime number within thebandwidth (8) of the data signal of user C is assigned to the leadingpilot signal 1 of user C, and a sequence length of 7 (N7) that is thelargest prime number within the bandwidth (8) of the data signal is alsoassigned to the succeeding pilot signal 2. In this case, it ispreferable to map the leading pilot signal 1 onto subcarriers startingfrom one end of the frequency band and to map the succeeding pilotsignal 2 onto subcarriers starting from the other end of the frequencyband as shown in FIG. 3 so that the leading pilot signal 1 and thesucceeding pilot signal 2 complement each other. With the exemplarypilot signal mapping shown in FIG. 3, the channel estimation for SC f25of the data signal of user C can be accurately performed by using thesucceeding pilot signal 2 (N7) of user C, and the channel estimation forSC f32 can be accurately performed by using the leading pilot signal 1(N7) of user C. As described above, embodiments of the present inventioneliminate the need to extrapolate channel estimates in the frequencydirection and thereby effectively prevent the degradation of accuracy ofchannel estimation.

FIG. 4 is a flowchart showing an exemplary process of determining thesequence length of a pilot signal performed by the sequence lengthdetermining units 41 and 71 according to embodiments of the presentinvention. In the exemplary process, the sequence length of a pilotsignal is determined so that the pilot signal uses the bandwidthcorresponding to that assigned to a data signal of the user as much aspossible and the pilot signal does not overlap another pilot signal ofan adjacent data signal (user). Also, the sequence length of a pilotsignal is determined so as not to exceed the system bandwidth.

In step S11, the sequence length determining unit 41 (the sequencelength determining unit 41 is used here for descriptive purpose) obtainsthe data signal bandwidth DBA assigned to user A. In step S12, thesequence length determining unit 41 sets the smallest prime numberexceeding the data signal bandwidth DBA in the wider pilot signalsequence length PAW (of the leading pilot signal 1). In step S13, thesequence length determining unit 41 sets the largest prime number withinthe data signal bandwidth DBA in the narrower pilot signal sequencelength PAN (of the succeeding pilot signal 2). In step S14, the sequencelength determining unit 41 determines whether the frequency band next tothat of the data signal of user A is assigned to a data signal ofanother user.

If the next frequency band is assigned to another user (user B), in stepS15, the sequence length determining unit 41 obtains the data signalbandwidth DBB assigned to user B. In step S16, the sequence lengthdetermining unit 41 sets the smallest prime number exceeding the datasignal bandwidth DBB in the wider pilot signal sequence length PBW (ofthe succeeding pilot signal 2). In step S17, the sequence lengthdetermining unit 41 sets the largest prime number within the data signalbandwidth DBB in the narrower pilot signal sequence length PBN (of theleading pilot signal 1).

In step S18, the sequence length determining unit 41 determines whetherthe sum of the sequence lengths (PAW and PBN) of the leading pilotsignals 1 of users A and B exceeds the sum of the data signal bandwidths(DBA and DBB). If the sum of the sequence lengths (PAW and PBN) exceedsthe sum of the data signal bandwidths (DBA and DBB), in step S19, thesequence length determining unit 41 obtains the exceeding (overlapping)amount Δ AB of the sum of the sequence lengths by the following formula:(PAW+PBN)−(DBA+DBB). In step S20, the sequence length determining unit41 replaces the narrower pilot signal sequence length PBN of user B withthe largest prime number within (PBN−Δ AB). If the sum of the sequencelengths (PAW and PBN) does not exceed the sum of the data signalbandwidths (DBA and DBB), steps 19 and 20 are skipped.

In step S21, the sequence length determining unit 41 determines whetherthe sum of the sequence lengths (PAN and PBW) of the succeeding pilotsignals 2 of users A and B exceeds the sum of the data signal bandwidths(DBA and DBB). If the sum of the sequence lengths (PAN and PBW) exceedsthe sum of the data signal bandwidths (DBA and DBB), in step S22, thesequence length determining unit 41 obtains the exceeding (overlapping)amount ΔBA of the sum of the sequence lengths by the following formula:(PAN+PBW)−(DBA+DBB). In step S23, the sequence length determining unit41 replaces the narrower pilot signal sequence length PAN of user A withthe largest prime number within (PAN−Δ BA) and exits the process. If thesum of the sequence lengths (PAN and PBW) does not exceed the sum of thedata signal bandwidths (DBA and DBB), steps 22 and 23 are skipped.

In step S14 described above, if the next frequency band is not assignedto another user, the sequence length determining unit 41 replaces thewider pilot signal sequence length PAW with the narrower pilot signalsequence length PAN and exits the process. There are two cases when theresult of step S14 becomes No. One case is that the system bandwidth isfully occupied by data signals. The other case is that there is afrequency band available in the system bandwidth but there is no userwho uses the frequency band. Step S24 is provided on the assumption thatthe system bandwidth is fully occupied. When there is a frequency bandavailable in the system bandwidth but there is no user who uses thefrequency band, the wider pilot signal sequence length PAW of user A maybe used without change. However, the wider pilot signal sequence lengthPAW must still be within the system bandwidth.

As described above, the exemplary process according to this embodimenthandles pilot signals of two users at a time and pilot signals of morethan two users can be easily mapped onto subcarriers by repeating theexemplary process. Even when one user is left after repeating theexemplary process, steps 14 and 24 makes it possible to appropriatelydetermine the sequence lengths of the pilot signals of the remaininguser.

FIGS. 5A through 7B are drawings illustrating other exemplary mappingsof pilot signals according to embodiments of the present invention. InFIG. 5A, the sequence lengths of pilot signals are determined based onRBs having the same bandwidth. In this example, RB1 through RB5 eachhaving a bandwidth of 12 are assigned to data signals of users A throughE, respectively. Also, pilot signals 1 and pilot signals 2 are placedbefore and after the data signals in the sub-frame.

When the bandwidth of each of RBs is 12, the largest prime number withinthe RB bandwidth is 11 and the smallest prime number exceeding the RBbandwidth is 13. These prime numbers are used as the sequence lengthsN11 and W13. In the example, W13 is assigned to the leading pilot signal1 of RB1 and N11 is assigned to the succeeding pilot signal 2. Thepropagation characteristic of the rightmost subcarrier of RB1 can beaccurately estimated by using the leading pilot signal 1 (W13).

N11 is assigned to the leading pilot signal 1 of RB2 that is adjacent toRB1 and W13 is assigned to the succeeding pilot signal 2. With thismapping scheme, the propagation characteristic of the leftmostsubcarrier of RB2 can be accurately estimated by using the succeedingpilot signal 2 (W13). Also, the leading pilot signals 1 of RB1 and RB2do not interfere with each other, and the succeeding pilot signals 2 ofRB1 and RB2 do not interfere with each other. Further, the sum of thesequence lengths W13 and N11 is 24 and equals the sum of the bandwidths(24) assigned to users A and B. The sequence lengths of pilot signals ofRB3 and RB4 can be determined in substantially the same manner asdescribed above.

In the case of RB5, the bandwidth assignable to each of the leading andsucceeding pilot signals 1 and 2 is limited to the data signal bandwidth(12) of RB5 alone. Therefore, N11 is assigned to each of the leading andsucceeding pilot signals 1 and 2. In this case, it is preferable to mapthe leading pilot signal 1 onto subcarriers starting from one end of RB5and to map the succeeding pilot signal 2 onto subcarriers starting fromthe other end of RB5 as shown in FIG. 5A. With this mapping scheme, thepropagation characteristic of the rightmost subcarrier of RB5 can beaccurately estimated by using the rightmost symbol of the leading pilotsignal 1 (N11), and the propagation characteristic of the leftmostsubcarrier of RB5 can be accurately estimated by using the leftmostsymbol of the succeeding pilot signal 2 (N11). Also, since the number ofsubcarriers where the leading pilot signal 1 or the succeeding pilotsignal 2 is not mapped is 1, its propagation characteristic may also beobtained by extrapolation.

In FIG. 5B, the sequence lengths of pilot signals are determined basedon data signal bandwidths assigned to users. In this example, abandwidth of 12 is assigned to user A, a bandwidth of 36 is assigned touser B, and a bandwidth of 12 is assigned to user C. Also, pilot signals1 and pilot signals 2 are placed before and after the data signals inthe sub-frame. In the case of user A with a data signal bandwidth of 12,W13 is assigned to the leading pilot signal 1 and N11 is assigned to thesucceeding pilot signal 2. The propagation characteristic of therightmost subcarrier of user A can be accurately estimated by using theleading pilot signal 1 (W13).

In the case of user B with a data signal bandwidth of 36, the largestprime number within the data signal bandwidth is 31 and the smallestprime number exceeding the data signal bandwidth is 37. These primenumbers are used as the sequence lengths N31 and W37. In the example,N31 is assigned to the leading pilot signal 1 of user B and W37 isassigned to the succeeding pilot signal 2. With this mapping scheme, thepropagation characteristics of the five leftmost subcarriers of user Bcan be accurately estimated by using the succeeding pilot signal 2(W37).

Also, the leading pilot signals 1 of users A and B do not interfere witheach other, and the succeeding pilot signals 2 of RB1 and RB2 do notinterfere with each other. Further, the sum of the sequence lengths N11and W37 of the succeeding pilot signals 2 is 48 and equals the sum ofthe bandwidths (48) assigned to users A and B. Although the sequencelength N31 of the leading pilot signal 1 of user B is 5 short of thedata signal bandwidth (36), the propagation characteristics of the fiveleftmost subcarriers of user B can be accurately estimated by using thesucceeding pilot signal 2 (W37). The leading pilot signal 1 (N31) ofuser B may also be mapped onto subcarriers in the middle of theavailable bandwidth so that leftmost and rightmost subcarriers are leftblank. The sequence lengths of pilot signals of user C can be determinedin substantially the same manner as in the case of RB5 shown in FIG. 5A.

In FIG. 6, each of the pilot signals that has a sequence length smallerthan the data signal bandwidth is mapped onto subcarriers starting fromthe left (lower) or right (higher) end of the data signal bandwidth. Adata signal bandwidth of 16 is assigned to each of users A and B. Inthis case, the largest prime number within the data signal bandwidth(16) is 13. In the case of user A, the leading and succeeding pilotsignals 1 and 2 each having a sequence length of N13 that is smallerthan the data signal bandwidth are mapped onto subcarriers starting fromthe left end of the data signal bandwidth and onto subcarriers startingfrom the right end of the data signal bandwidth, respectively. Thepropagation characteristics of SCs f1 through f3 can be accuratelyestimated by using the leading pilot signal 1 (N13) and the propagationcharacteristics of SCs f14 through f16 can be accurately estimated byusing the succeeding pilot signal 2 (N13). The succeeding pilot signal 2(N13) may also be used as the leading pilot signal 1 for the channelestimation of the next data block.

In this example, the leading and succeeding pilot signals 1 and 2 ofuser B are mapped onto subcarriers in a manner opposite to that of userA. However, the leading and succeeding pilot signals 1 and 2 of user Bmay be mapped in the same manner as in the case of user A. With theabove mapping scheme, the propagation characteristics of SCs f17 throughf19 can be accurately estimated by using the succeeding pilot signal 2(N13) and the propagation characteristics of SCs f30 through f32 can beaccurately estimated by using the leading pilot signal 1 (N13). Eitherpilot signals having the same sequence number k or pilot signals havingdifferent sequence numbers k may be used for users A and B.

In FIGS. 7A and 7B, the bandwidths of data signals are adjusted to equalthe sequence lengths of pilot signals. In FIG. 7A, the sequence lengthsof pilot signals are determined based on RBs. When the bandwidth of eachof RBs is 12, the smallest prime number exceeding the RB bandwidth is 13and the largest prime number within the RB bandwidth is 11. In thiscase, data signal bandwidths of 13 and 11 are assigned to users A and B,respectively. Also, sequence lengths of W13 and N11 are assigned to thepilot signals of users A and B, respectively. This makes it possible toefficiently use the sum of the bandwidths (13+11=24) of RB1 and RB2.Also, this mapping scheme makes it possible to use all the symbols ofpilot signals for channel estimation. RB3 and RB4 can be handled insubstantially the same manner as described above. For the last one (RB5)of RBs, pilot signals with a sequence length of N11 are used.

In FIG. 7B, the sequence lengths of pilot signals are determined basedon data signal bandwidths assigned to users. When the data signalbandwidth that is normally assigned to each of users A and B is 24, thesmallest prime number exceeding the data signal bandwidth is 29 and thelargest prime number within the data signal bandwidth is 23. The sum ofthe prime numbers becomes 52. In this case, data signal bandwidths of 29and 23 are assigned to users A and B, respectively. Also, sequencelengths of W29 and N23 are assigned to the pilot signals of users A andB, respectively. This makes it possible to efficiently use the bandwidthof 52 for the pilot signals and the data signals without leaving space.Meanwhile, it is also possible to assign a data signal bandwidth of 23to user A and a data signal bandwidth of 29 to user B. Also, a datasignal bandwidth of 7 (out of the remaining bandwidth of 8) may beassigned to user C (not shown).

FIG. 8 and FIG. 9 are drawings illustrating another exemplary mobilecommunication system according to a second embodiment of the presentinvention. In this exemplary mobile communication system, priorityinformation for determining the sequence length of pilot signals is usedin addition to RB assignment information in order to more efficientlyand flexibly perform pilot signal mapping. FIG. 8 is a block diagramillustrating an exemplary configuration of an exemplary receiving unitof a base station 10. The exemplary receiving unit of the base station10 includes, in addition to the components shown in FIG. 1, a deficiencyrate calculation unit 42 that calculates the deficiency rate of abandwidth assignable to a pilot signal (pilot signal bandwidthdeficiency rate); a priority determining unit 43 that determines thepriority used to determine the sequence length of a pilot signal basedon the deficiency rate and generates priority information; and a buffer30 b that retains (delays) the next priority information to be fed backto a mobile station 50 so that the next priority information can be usedas the current priority information used to process a subframe signalreceived from the mobile station 50.

FIG. 9 is a block diagram illustrating an exemplary configuration of anexemplary transmitting unit of the mobile station 50. The exemplarytransmitting unit of the mobile station 50 includes, in addition to thecomponents shown in FIG. 2, a sequence length determining unit 71 thatdetermines a sequence length (bandwidth) of a pilot signal based on thenext RB assignment information and the next priority information sentfrom the base station 10.

The priority determining unit 43 shown in FIG. 8 generates priorityinformation, which is used to determine the sequence length of a pilotsignal, for each of the data signal bandwidths assigned to users. Thepriority information is basically determined based on the deficiencyrate of a bandwidth usable by a pilot signal (pilot signal bandwidthdeficiency rate) in relation to the bandwidth of a data signal.

FIGS. 10A and 10B are drawings used to describe the deficiency rate of abandwidth usable by a pilot signal. When BWdata is the bandwidth of adata signal and BWpilot is the bandwidth of a pilot signal, a deficiencyrate DR of the bandwidth usable for the pilot signal can be obtained bythe following formula (4):

$\begin{matrix}{{DR} = {\frac{{BW}_{data} - {BW}_{pilot}}{{BW}_{data}} = {1 - \frac{{BW}_{pilot}}{{BW}_{data}}}}} & (4)\end{matrix}$

The largest prime number within the bandwidth of a data signal differsdepending on the bandwidth of the data signal. A deficiency rate is usedas a measure of the largest prime number.

Exemplary prime numbers (2-59) are shown in FIG. 10B. As shown in FIG.10B, it can be assumed that the distribution of prime numbers in a setof natural numbers is substantially even. If so, it can be assumed thatthe deficiency rate of a bandwidth to be assigned to a pilot signal issubstantially constant regardless of the number of RBs. Therefore, thedeficiency rate of a bandwidth to be assigned to a pilot signal is ininverse proportion to the bandwidth of a data signal and can beapproximated by the following formula (5):

$\begin{matrix}{{DR} \propto \frac{1}{{BW}_{data}}} & (5)\end{matrix}$

In the exemplary range of prime numbers (2-59), the proportionalityconstant is between 2 and 6 (3.6 on average).

The priority determining unit 43 of the base station 10 sets thepriority of a user (mobile station) with a high deficiency rate to high,the priority of a user with a medium deficiency rate to middle, and thepriority of a user with a low deficiency rate to low. Such aprioritization method makes it possible to effectively prevent seriousdegradation of reception characteristics at the base station 10.

The mobile station 50 receives RB assignment information and priorityinformation from the base station 10. When the priority is high, themobile station 50 multiplexes a pilot signal (W) having a sequencelength (bandwidth) that is larger than the bandwidth assigned to thedata signal. When the priority is low, the mobile station 50 multiplexesa pilot signal (N) having a sequence length that is smaller than thebandwidth assigned to the data signal. When the priority is middle, themobile station 50 alternately multiplexes a pilot signal (W) and a pilotsignal (N). In other words, a user with a higher deficiency ratetransmits a pilot signal having a wider bandwidth. Such a pilot signaltransmission method makes it possible to effectively prevent seriousdegradation of the accuracy of channel estimation.

Also, the priority determining unit 43 is preferably configured toperform the priority determining process as described below taking intoaccount the actual communication environment. For example, when thepriorities of adjacent mobile stations are both high or both low, thepriority determining unit 43 resets both of the priorities to middle.This makes it possible to impartially assign bandwidths to the mobilestations. When there is a possibility of pilot signal interferencebetween adjacent mobile stations, the priority determining unit 43resets the priority of one of the mobile stations that has a higherpriority (high or middle) to low. This makes it possible to effectivelyprevent pilot signal interference. Also, the priority determining unit43 sets the priority of a mobile station which is assigned a frequencyband at an end of the system bandwidth to low. This makes it possible tosafely prevent increase of adjacent frequency band emission. Further,the priority determining unit 43 sets the priority of a mobile stationhaving a low modulation level to middle or low. Normally, acommunication using a channel with a low modulation level showscomparatively good reception characteristics (such as low BER).Therefore, lowering the priority of a mobile station having a lowmodulation level does not greatly degrade the reception quality. This,in turn, makes it possible to increase the priority of an adjacent user.

In the above embodiments, an uplink communication in a mobilecommunication system is used as an example. However, the presentinvention may also be applied to other aspects of radio communication.For example, a pilot signal multiplexing method according to anembodiment of the present invention may also be applied to downlinkcommunication. In the above embodiments, the sequence length and mappingof a pilot signal are determined at each mobile station based on RBassignment information from the base station. In other words, the RBassignment information is shared by the base station and the mobilestation. However, a mobile communication system may be configured sothat the base station determines the sequence length and mapping of apilot signal for a mobile station and sends the information to themobile station.

In the above embodiments, pilot signals are time-division multiplexedand placed at the top and bottom of a subframe. However, a differenttime-division multiplexing method may be used. For example, pilotsignals may be arranged at certain intervals in a subframe.

Also, in the above embodiments, sequence lengths of pilot signals of twousers are determined in one cycle of the exemplary sequence lengthdetermining process based on the combined bandwidth assigned to the twousers. However, the present invention is not limited to the aboveembodiments. For example, the exemplary sequence length determiningprocess may be configured to determine the sequence lengths of pilotsignals of three users in one cycle. Further, in the above embodiments,methods of arranging signals within a system bandwidth are described.However, the present invention may also be applied to methods ofarranging signals within the bandwidth of a cell.

As described above, embodiments of the present invention eliminate theneed to extrapolate channel estimates in the frequency direction andthereby make it possible to obtain accurate channel estimates. Also,embodiments of the present invention make it possible to improve theflexibility in assigning pilot signals with low cross-correlation toeach cell in a multicell environment.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on Japanese Priority Application No.2006-290881 filed on Oct. 26, 2006 with the Japanese Patent Office, theentire contents of which are hereby incorporated herein by reference.

1. A method of transmitting pilot signals in a mobile communicationsystem, comprising the steps of: time-division multiplexing a firstpilot signal and a second pilot signal for channel compensation togetherwith a data signal of a user, which data signal is assigned a certainbandwidth and to be wirelessly transmitted based on DFT-spread-OFDM,into a time-division multiplexed signal; frequency-division multiplexingthe time-division multiplexed signal together with time-divisionmultiplexed signals of other users into a frequency-division multiplexedsignal; and transmitting the frequency-division multiplexed signal;wherein the first pilot signal is assigned a bandwidth larger than thebandwidth assigned to the data signal and the second pilot signal isassigned a bandwidth smaller than the bandwidth assigned to the datasignal; and the first pilot signal and the second pilot signal aremultiplexed along a time axis.
 2. The method as claimed in claim 1,wherein each of the first pilot signal and the second pilot signal iscomposed of a sequence having a sequence length that is a limitednatural number.
 3. The method as claimed in claim 2, wherein a sequencelength of the first pilot signal is a smallest prime number exceedingthe bandwidth assigned to the data signal; a sequence length of thesecond pilot signal is a largest prime number within the bandwidthassigned to the data signal; and the first pilot signal and the secondpilot signal are multiplexed alternately along the time axis.
 4. Themethod as claimed in claim 3, wherein the first pilot signal and thesecond pilot signal are multiplexed in such a manner that either of thefirst pilot signal and the second pilot signal does not overlap a pilotsignal transmitted at substantially the same timing from an adjacent oneof the other users a data signal of which adjacent one of the otherusers is assigned a frequency band adjacent to that assigned to the datasignal of said user.
 5. The method as claimed in claim 4, wherein thedata signal of said user and the data signal of the adjacent one of theother users are assigned the same bandwidth.
 6. The method as claimed inclaim 4, wherein the data signal of said user and the data signal of theadjacent one of the other users are assigned different bandwidths.
 7. Amethod of transmitting pilot signals in a mobile communication system,comprising the steps of: time-division multiplexing a first pilot signaland a second pilot signal for channel compensation together with a datasignal of a user, which data signal is assigned a certain bandwidth andto be wirelessly transmitted based on DFT-spread-OFDM, into atime-division multiplexed signal; frequency-division multiplexing thetime-division multiplexed signal together with time-division multiplexedsignals of other users into a frequency-division multiplexed signal; andtransmitting the frequency-division multiplexed signal; wherein each ofthe first pilot signal and the second pilot signal is composed of asequence having a sequence length that is a largest prime number withinthe bandwidth assigned to the data signal; the first pilot signal isassigned a frequency band starting from one end of the bandwidthassigned to the data signal; the second pilot signal is assigned afrequency band starting from the other end of the bandwidth assigned tothe data signal; and the first pilot signal and the second pilot signalare multiplexed alternately along a time axis.
 8. A method oftransmitting a pilot signal in a mobile communication system, comprisingthe steps of: time-division multiplexing a pilot signal for channelcompensation together with a data signal of a user, which data signal isassigned a certain bandwidth and to be wirelessly transmitted based onDFT-spread-OFDM, into a time-division multiplexed signal;frequency-division multiplexing the time-division multiplexed signaltogether with time-division multiplexed signals of other users into afrequency-division multiplexed signal; and transmitting thefrequency-division multiplexed signal; wherein the pilot signal iscomposed of a sequence having a sequence length that is a prime number;and the bandwidth assigned to the data signal is adjusted to match abandwidth corresponding to the sequence length.
 9. A mobilecommunication system including a base station and at least one mobilestation where a pilot signal for channel compensation is time-divisionmultiplexed together with a data signal of the mobile station, whichdata signal is assigned a certain bandwidth and to be wirelesslytransmitted between the base station and the mobile station based onDFT-spread-OFDM, into a time-division multiplexed signal, thetime-division multiplexed signal is frequency-division multiplexedtogether with time-division multiplexed signals of other mobile stationsin the mobile communication system into a frequency-division multiplexedsignal, and the frequency-division multiplexed signal is transmitted,wherein the base station is configured to transmit information over adownlink control channel to the mobile station which informationincludes a bandwidth to be assigned to the data signal of the mobilestation; the mobile station is configured to multiplex a first pilotsignal composed of a sequence having a sequence length that is asmallest prime number exceeding the bandwidth assigned to the datasignal of the mobile station and a second pilot signal composed of asequence having a sequence length that is a largest prime number withinthe bandwidth assigned to the data signal of the mobile stationalternately along a time axis in such a manner that either of the firstpilot signal and the second pilot signal does not overlap a pilot signaltransmitted at substantially the same timing from an adjacent one of theother mobile stations a data signal of which adjacent one of the othermobile stations is assigned a frequency band adjacent to that assignedto the data signal of the mobile station.
 10. A mobile communicationsystem including a base station and at least one mobile station where apilot signal for channel compensation is time-division multiplexedtogether with a data signal of the mobile station, which data signal isassigned a certain bandwidth and to be wirelessly transmitted betweenthe base station and the mobile station based on DFT-spread-OFDM, into atime-division multiplexed signal, the time-division multiplexed signalis frequency-division multiplexed together with time-divisionmultiplexed signals of other mobile stations in the mobile communicationsystem into a frequency-division multiplexed signal, and thefrequency-division multiplexed signal is transmitted, wherein the basestation is configured to transmit information over a downlink controlchannel to the mobile station which information includes a bandwidth tobe assigned to the data signal of the mobile station and a priority todetermine a sequence length of the pilot signal; and the mobile stationis configured to, when the priority is high, multiplex a first pilotsignal composed of a sequence having a sequence length that is asmallest prime number exceeding the bandwidth assigned to the datasignal of the mobile station.
 11. The mobile communication system asclaimed in claim 10, wherein, when the priority is low, the mobilestation multiplexes a second pilot signal composed of a sequence havinga sequence length that is a largest prime number within the bandwidthassigned to the data signal of the mobile station.
 12. The mobilecommunication system as claimed in claim 11, wherein, when the priorityis middle, the mobile station multiplexes the first pilot signal and thesecond pilot signal alternately along a time axis in such a manner thateither of the first pilot signal and the second pilot signal does notoverlap a pilot signal transmitted at substantially the same timing froman adjacent one of the other mobile stations a data signal of whichadjacent one of the other mobile stations is assigned a frequency bandadjacent to that assigned to the data signal of the mobile station. 13.The mobile communication system as claimed in claim 12, wherein the basestation is configured to obtain a pilot signal bandwidth deficiency rateof the mobile station by the following formula and, when the pilotsignal bandwidth deficiency rate is high, to set the priority of themobile station to high: (bandwidth of data signal−bandwidth of pilotsignal)/(bandwidth of data signal).
 14. The mobile communication systemas claimed in claim 13, wherein, when the pilot signal bandwidthdeficiency rate is low, the base station sets the priority of the mobilestation to low.
 15. The mobile communication system as claimed in claim14, wherein, when the priority of the mobile station and the priority ofthe adjacent one of the other mobile stations are both high or both low,the base station resets both of the priority of the mobile station andthe priority of the adjacent one of the other mobile stations to middle.16. The mobile communication system as claimed in claim 15, wherein,when there is a possibility of pilot signal interference between themobile station and the adjacent one of the other mobile stations, thebase station resets one of the priorities of the mobile station and theadjacent one of the other mobile stations to low which one of thepriorities is high.
 17. The mobile communication system as claimed inclaim 15, wherein the base station is configured to set the priority ofthe mobile station to low when a frequency band assigned to the datasignal of the mobile station is located at an end of a bandwidthassigned to the mobile communication system.
 18. The mobilecommunication system as claimed in claim 15, wherein the base station isconfigured to set the priority of the mobile station to low when amodulation level of the mobile station is low.